6 TH-53

Proceedings of Indian Geotechnical Conference

December 22-24,2013, Roorkee

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SOIL IMPROVEMENT USING ACCELERATED CONSOLIDATION

TREATMENT TO IMPROVE LOAD BEARING OF SOFT SOIL

S. Sahu ,* Manager (India), Menard Asia, New Delhi, [email protected]

P.J.Rao,* Consulting Engineer, Faridabad, [email protected]

K.Yee, * Regional Director, Menard Asia, KaulaLampur, [email protected]

ABSTRACT: Roads constructed on high embankments underlain by soft strata are susceptible to long term

settlements which may lead to the recurring maintenance requirements. The utilization of conventional method of

removal and replacement of unsuitable material with suitable compacted fill for embankment construction over

shallow marginal ground has been common practice in India, in the past. Due to the need for speedy and

economical construction, and current stringent environmental requirements, more options need to be considered. In

many such cases, Ground Improvement proved to be a good solution by modifying the soil characteristics with or

without the addition of imported materials.

The paper describes the case study of a project executed in Malaysia where the height of the embankment varies

from 5m to 16m. It was initially envisaged to remove and replace the unsuitable materials to depth up to 6m with

ground water table at the surface. Due to construction constraints, cost of fill materials and the non-favorable

environmental impact assessment (EIA), an alternative ground improvement solution using a combination of

dynamic replacement (DR) and prefabricated vertical drains (PVD) was carried out to provide the necessary

foundation treatment for the road embankment.

INTRODUCTION

Putrajaya Road network in Malaysia involved

construction of an embankment on marginal

ground. The height of the embankment varies from

5m to 16m. It was envisaged to remove and replace

the unsuitable materials to a depth up to 6m with

ground water table at the surface. Due to

construction constraints, cost of fill materials and

the non-favorable environmental impact

assessment (EIA), an alternative ground

improvement solution using a combination of

dynamic replacement (DR) and prefabricated

vertical drains (PVD) was carried out to provide

the necessary foundation treatment for the road

embankment.

PRINCIPLE OF DYNAMIC REPLACEMENT

AND VERTICAL DRAINS

Dynamic replacement (DR) is a ground

reinforcement technique whereby stiff elements

are introduced as inclusions in the ground by heavy

tamping with the main benefit resulting from the

structural aspect of the elements themselves. A

composite soil-element structure, interacting

through friction and adhesion, increases the

bearing capacity, improves stability and reduces

settlement. Inclusion materials are granular

materials such as sand, aggregate, stone and/or

rock pieces (up to 300mm size).The technique

starts out by creating a crater with light tamping.

The craters are then backfilled with granular

materials that will lock together under subsequent

heavy tamping. This technique essentially results

in large columns of compacted granular material

up to 2.53m in diameter. Because of the higher

permeability of this granular backfill material,

pore-water pressure from the underlying and

surrounding soft soils will dissipate quickly.

Hence, DR columns besides being load bearing

columns (with bearing capacity even up to 80 tons)

also serve as large vertical granular drains. Since

spacings of DR columns are typically between 4m

to 7m, prefabricated vertical drains (PVD) were

installed between columns to further speed up

therate of consolidation. Unlike DR columns, PVD

do not serve any structural function. A typical

dynamic replacement process is show in Figure 1.

S. Sahu, P. J. Rao & K. Yee

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Fig. 1 Dynamic Replacement Process

GROUND CONDITIONS

The site was located in a low-lying broad valley

between low hills. Ground water table was about

0.3m below surface. Exploratory boreholes and

cone penetration tests (CPT) (Figure 2) were

carried out which indicated an upper layer of very

soft soils varied from46m thick with organic peat

interspersed in places up to 2m thick; overlying

stiff silt/clay layer down to refusal depth at about

11m. A summary of soil properties is given in

Table 1.

Field vane shear tests were carried out and the

results are shown in Figure 3. The Suvane values

were corrected according to Bjerrum (1972) with

correction factor l = 0.8 based on plasticity index

of 50. The average Su= 13 kN/m2 was used and it

is compared with the average CPT qc= 0.23MPa

for the soft layer which corresponds to Su= 15

kN/m2.

Fig: 2 Cone penetration test result

Fig. 3 Field vane shear test result

Table 1: Ground Condition and SPT Results

Depth Material SPT N Atterberg

Limits

UU Triaxial

compression

1-D

Consolidation

0-4 m Very soft silty clay with

organic material (peat) and

traces of sand

0 Wn ~ 75%

LL ~ 79%

PL ~ 29%

PI ~ 50%

C ~ 9-13kN/m2

= 0o

Pc ~ 36-50 kPa

Cc ~ 0.43-0.7

4-5 m Soft clayey silt 2-4

5-11 m Firm sandy silt /

stiff silty clay

6-11

> 24

Soil Improvement using accelerated consolidation treatment to improve load bearing of soft soil

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The coefficient of vertical consolidation (Cv)

varied from 1.17 to 1.91m2/y with an average of

1.5m2/y. Based on the liquid limit of

79%,NAVFAC(1982) suggests a Cv= 1.36m2/y.

For design, Cv= 1.5m2/y and Ch= 3Cv= 4.5m

2/y

(Chbeing the coefficient of horizontal

consolidation) were adopted. The CPT dissipation

test results suggested an in-situ Ch= 4.1 to 5.1m2/y

based on Houlsby&Teh (1988).

PERFORMANCE CRITERIA

The height of the embankment varies from 5m to

16m with a crest width of 49m to 250m, wider

towards the interchange. The slope was at 1V:2H

with intermediate berm at every 6m height.

The performance criteria are as follows:

a) Total settlement within the first 7 years of

service shall not exceed 10%of the sum of the total

theoretical primary settlement and secondary

settlement, the later being assessed for a period of

20 years;

b) Settlement within the first 7 years of service

shall nowhere exceed 400mm;

c) In areas of transition between piled approach

embankments and general low embankments,

differential settlement within the first 7 years of

service shall not exceed 100mm within a length of

50m. In areas remote from structures and transition

zones, differential settlement shall not exceed

100mm within a length of 100m.

d) The factor of safety against instability during

construction shall not be less than 1.2 and after

construction not less than 1.5.

DESIGN SCHEME

Since the construction period was limited to 12

months, in order to achieve the required

performance criteria in such a short duration it

became important to accelerate the consolidation

process which was not possible to achieve just by

placing surcharge or using only PVDs with

surcharge. A combination of DR columns and

PVD is deemed necessary. The required drainage

is provided by the DR columns and PVD. In

addition, the DR columns also serve as a bearing

support to the embankment so that the required rate

of filling of embankment can be achieved with

adequate support of the embankment by its

foundation during the construction stage as well as

end-of-construction stage. Without DR columns,

the rate of filling as well as the final height of the

embankment may be limited.A closer grid of DR

columns and PVD was constructed below the slope

of the embankment to provide edge reinforcement

for stability. It followed by a wider grid of DR

columns and PVD constructed below the crest of

the embankment for normal bearing support.

FIELD CALIBRATION

A field calibrationwas carried out on an area of

500m2 to determine the operational parameters

(e.g. nos. of blows, drop height, etc.) and to

validate design assumptions prior to

commencement of full production works. PVDs

were installed to an average depth of 56m

followed by the construction of the DR test

columns.

Length of DR columns varied from 4m to 5m. Pre

and post treatment pressuremeter tests (PMT) and

CPT were carried out inside the DR columns and

in-between the columns. The post treatment tests

were carried out after 14 days. The characteristic

PMT and CPT values are as given in Table 2.

Table 2: Characteristic PMT and CPT Values

Description

PMT CPT

Limit

Pressure

PL

Pressure-

meter

modulus

Cone

Resistance

qc

Inside DR

Columns 1070 kPa 7500 kPa 9MPa

In-between

DR Columns 620 kPa 3950 kPa 2MPa

FIELD TESTING IMPLEMENTATION &

EVLUATION

The depth of treatment, as defined by the

pretreatment CPT and PMT results; followed by

installation of PVD and DR works; and post

treatment PMT for quality control and assurance.

The total treatment area was about 102,000m2. A

total of 78 CPTs and 36 PMTs were carried out

prior to PVD and DR works. The post-treatment

PMT tests were carried out, 14 days after the

completion of works but, before commencement of

embankment filling. During embankment


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construction, settlement readings from settlement

plates placed at 50m distance were analyzed using

Asoaka (1978) method to predict the final

settlement. Lateral deformations from

inclinometers were analyzed usingMatsuo &

Kawamura (1977) method for stability assessment.

The criterion of b < 0.25 being the ratio of lateral

to vertical movement was used for embankment

stability control.

Settlement analysis was carried out based on Eq.

(1) and (2) given by Terzaghi (1967). The soil was

assumed to be normally consolidated.

(

)

(

)

(2)

Where, Cr= recompression index; Cc= compression

index; pc= maximum past pressure; = overburden effective stress; = final pressure; eo= initial void ratio and LL = liquid limit.

Based on the above equations, the computed

settlement without DR columns (SW/ODR) varies

from 95 cm to 150 cm for embankment height

ranging from 5m to 16m. With the DR columns,

the stiffness modulus of the composite soil-column

system is increased and hence, settlement is

reduced. The measured settlement (SDR) obtained

from settlement plates placed along the centreline

of the embankment ranged from 32 cm to 65 cm.

Thus, it is evidenced that the DR columns have

reduced the settlement and improved the

performance of the subsoil in comparison to the

state without DR columns.

Further the field results were compared with the

Priebe (1995) predictions of settlement defined by

an improvement factor n (=SW/ODR/ SDR). The

friction angle of the DR column material was

estimated from the limit pressure, PL (Centre

dEtudes, Menard 1970) measured inside the DR

pillar. The computation for improvement factor n

is given in Table 3.

Maximum settlement of 65 cm was recorded at

chainage 500 (i.e. about center of treatment area)

where the soft layer was at maximum 6m thick and

the embankment height was at 8.4m. The

settlement characteristic is shown in Figure 4. The

computed settlement without DR columns is about

135 cm. Hence, the improvement factor n is 2.07

compared with 1.96 as computed. Hence, the

Priebe (1995) method can provide a reasonable

estimation of settlement reduction with DR

columns in this project.

Table 3: Computation for improvement factor n

DR Columns

Column diameter 2.9 m

Grid spacing (square) 5.5 m

Replacement Ratio (m) 22%

Friction Angle (c) 34o

Area of Column (Ac) 6.61 m2

Grid Area (A) 30.3 m2

Coef. of earth pressure

(KAC)

0.28

F (s, Ac/A) 0.95

Improvement factor n 1.96

The rate of settlement reduced from a peak of

about 10mm per day during the early stage of

construction to 0.1mm per day at later stage of

construction. The ratio of lateral to vertical

movement (b = dhor/dver) was highest at 0.22 during

the early stage of filling and reduced quickly to

0.03 after much consolidation has occurred. The

maximum total lateral movement recorded was

about 13cm at the highest embankment.

BENEFICIAL EFFECTS

Since the rate of consolidation is affected by the

available drainage facilities and the rate of filling

of embankment, the main beneficial effect of using

DR columns in combination with PVD is that the

rate of filling of embankment can be accelerated as

illustrated in Figure4. During the early stage of

construction, 2.5m of fill was placed over a period

of 14 days and 4m of fill was completed in 30

days, averaging about 1m per week. Without DR

columns, the rate of filling may be limited to the

Soil Improvement using accelerated consolidation treatment to improve load bearing of soft soil

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Fig 4: Settlement Characteristics at Ch. 500

bearing capacity of the soft soil, typically at about

0.4m0.5mperweek evenwith PVD. Also, the

increased rate of consolidation is evidenced by the

gradient of the time-settlement curve. The rate of

settlement (mm/day) shows that most of the

settlement has been achieved during construction

stage. DR columns and the corset effect provided

the necessary stability. Stability analyses indicated

factors of safety greater than 1.25 at all stages of

construction. During construction, lateral stability

with ratio b < 0.25 was maintained. Hence, the

beneficial effect of DR columns to serve as load

bearing columns and providing vertical drainage in

combination with PVD especially during the early

stages of construction has been demonstrated here.

CONCLUSION

The design, implementation and performance of a

combination of dynamic replacement columns and

prefabricated vertical drains for the construction of

a high embankment on marginal ground are

reported. The beneficial effects were demonstrated

through the accelerated built-up of the

embankment, accelerated rate of consolidation,

reduced postconstruction settlement; stability of

the embankment during construction as well as end

of construction and the reduced carbon footprint

compared with the exhibited method of removal

and replacement. This case history has highlighted

a ground improvement scheme that is suitable for

the construction of high embankment founded on


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shallow deposit of soft soils with time constraint

and promotes sustainable construction.

REFERENCES

1. Bjerrum L. (1972), Embankments on Soft Ground, Proceedings Specialty Conference on

Performance of Earth and Earth Supported

Structures, ASCE, Vol. 2, pp. 154.

2. NAVFAC (1982)., Design Manual 7.01 Soil Mechanics, Alexandria, V.A. Department of

the Navy, Naval Facilities Engineering

Command

3. Houlsby G. T. and Teh C. I. 1988), Analysis of the Piezocone in Clay, Penetration Testing

1988, ISOPT-1,ed. De Ruiter, Balkema (Pubs.),

Rotterdam

4. Asaoka A. (1978), Observational Procedure of Settlement Prediction, Journal of Soils and

Foundations 18(4) 87101.

5. Matsuo M. and Kawamura K. (1977), Diagram for Construction Control of Embankment on

Soft Ground, Journal of Soils and Foundations

17(3) 3752.

6. Priebe H. J. (1995), The Design of Vibro Replacement, Ground Engineering, pp. 3137.

7. Centre dEtudes Menard, Determination de la PousseeExercee par un Sol Su rune Paroi de

Sourenement (1970), Publication D/38/70.

8. Lambe T. W. and Whitman R. V. (1968), Soil Mechanics JohnWiley& Sons (Pubs.), New

York, p. 553.

9. Terzaghi K. and Peck R. B. (1967), Soil Mechanics in Engineering Practice: 2nd

edition, JohnWiley and Sons (Pubs.), New

York, p. 729.

6 TH-53

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